Pyrolysis converts urban pruning waste into biochar with soil and climate benefits

Urban pruning waste represents an environmental challenge for cities, and its valorization through biochar production in continuous reactors at the industrial scale has scarcely been explored. This study evaluated the conversion of residues in a horizontal pilot-scale continuous reactor. Four biochars (BC1–BC4) were produced under different operational conditions, including various particle sizes (< 10 to < 90 mm), inlet temperatures (140–300 °C), average operating temperatures (290–350 °C), and residence times (60–70 min). All the samples were characterized by their elemental composition (CHN/O), whereas BC4 was selected for more comprehensive analyses, including potentially toxic metals and organic contaminants, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDS), surface area determination (BET), and thermogravimetric analysis (TGA) under air, and recalcitrance index (R50). The carbon sequestration potential was estimated from the Corg and H/Corg ratio. The biochars displayed distinct characteristics. BC1 and BC4 presented the highest carbon contents (71.63% and 70.80%, respectively) and the lowest H/C and O/C ratios. BC3 had the lowest carbon content (47.38%) and the highest ash fraction (38.61%). BC4 had a high degree of carbonization, low ash content (15.40%), favorable atomic ratio (H/Corg = 0.45; O/Corg = 0.13), alkaline pH (8.77), and low specific surface area (4.58 m² g−1). It also complied with European Biochar Certificate thresholds for metals and organic contaminants. Thermal analysis confirmed moderate recalcitrance (R50 = 0.5), and its estimated carbon sequestration potential was 1.62 t CO₂e t−1 biochar. Overall, urban pruning waste biochar has proven safe, stable, and potentially useful for soil improvement and climate change mitigation, although feedstock heterogeneity and logistics remain challenges for large-scale implementation.